The present invention generally relates to the field of wireless communications equipment and processes for the deaf or speech impaired and, more specifically, to the field of TDD and/or TTY devices used by the deaf or speech impaired to communicate. In particular, the invention provides a wireless communication device that can transmit TTY/TDD messages at variable data rate.
There is an immediate need to provide enhanced text capabilities for the deaf, hearing impaired, and/or speech impaired communities. This is consistent with an overriding social goal of integrating handicapped individuals into traditional society, which is, in part, perhaps, best illustrated by the two recent statutes passed by the United States Congress and signed by the President of the United States: the Americans with Disabilities Act (xe2x80x9cADAxe2x80x9d) and the Telecommunications Act of 1996. Among other things, these statutes compel manufactures and telecommunications service providers to enhance existing approaches used by members of the deaf, hearing impaired, and/or speech impaired communities to access and utilize telecommunication networks and systems. For instance, the ADA generally requires that handicapped persons have equal access to public (and, in some cases, private) facilities. Among other things, 47 U.S.C. sctn.225 (b)(1) states that the Federal Communications Commission xe2x80x9cshall ensure that interstate and intrastate telecommunications relay services are available, to the extend possible and in the most efficient manner, to hearing-impaired and speech-impaired individuals in the United States.xe2x80x9d Consequently, over the past several years, it has become imperative for telecommunications companies to develop systems, processes, and apparatus that enable members of the deaf, hearing impaired, and/or speech impaired communities to use telecommunications networks and systems.
Deaf, hearing impaired, and/or speech impaired individuals primarily communicate with others via a Telecommunication Device for the Deaf (xe2x80x9cTDDxe2x80x9d), which is primarily a teletypewriter (xe2x80x9cTTYxe2x80x9d). One current problem for users of TTY/TDD devices is that the number of people who can be called is limited, because TTY/TDD users can only have conversations with others who have a TTY/TDD device, have access to a TTY/TDD device, and know how to use a TTY/TDD device.
Compounding this problem is the fact that most TTY/TDD devices currently available in the marketplace are land based and the user must have access to a standard telephone line to be able to communicate. Clearly, it would be desirable to develop wireless TTY/TDD devices that can allow the communication capabilities of the deaf, hearing impaired or speech impaired to be extended in the wireless world.
One problem with designing new systems, processes, and devices, particularly of wireless nature is that TTY/TDD devices are, by today""s standards, quite slow. Specifically, TTY/TDD uses the Baudot format consisting of 5-bit characters transmitted at a rate as slow as 45.45 bits per second, which is a TTY/TDD standard. These signals do not have any carrier and only support 38 characters. Numbers and punctuation characters are generally sent after a shift character is sent to change the character mode to a number mode, so as an example, the number xe2x80x9c3xe2x80x9d and the letter xe2x80x9cExe2x80x9d are both represented by the same code (00001).
To make the digital wireless technology accessible to TTY/TDD users it has been recently suggested to modify the existing IS-127 and IS-733 digital communication standard to allow TTY/TTD messages to be transmitted by using audio data frames as a vehicle. Typically, an audio data frame comprises an excitation segment and a coefficients segment. The coefficients segment holds a set of LPC or LSF (xe2x80x9cLinear Prediction Coefficientsxe2x80x9d, xe2x80x9cLine Spectral Frequenciesxe2x80x9d) coefficients while the excitation segment contains codebook entries, pitch lag information and codebook gain values. When the wireless communication system is used to transport speech sound information the audio data frames sent over the wireless channel are generated at the transmitter from PCM (xe2x80x9cPulse Code Modulationxe2x80x9d) samples that are processed by an LPC encoder. The reverse operation takes place at the receiver where the audio data frames are converted into PCM samples by an LPC decoder. When a TTY/TDD message is to be sent, the audio data frame is assembled somewhat difficulty. The TTY message that is a succession of characters is represented by the PCM stream as a series of tones. The encoder is designed to recognize the presence of the tones in the PCM stream. When tones are detected, the coefficients segment is computed as usual and a particular code that represents the TTY character is inserted into the pitch lag information field of the audio data frame. In addition, the adaptive codebook gain information field is set to a value that indicates a no gain condition. Actually, the TTY/TDD character is sent over the audio data frames in two different formats. The first format is the code representing the character and placed into the pitch lag information field while the second format is the tones information sent into the coefficients segment. In light of the fact that the tones are generated at a low bit rate, several consecutive audio data frames are required to contain the tone information representing a single character.
The two different formats are used primarily for reasons of backward compatibility. The new style decoders designed to detect the TTY/TDD character tones sent into the pitch lag information field only rely on that information to re-regenerate the tones at the receiver, while older style decoders rely on the FCB (xe2x80x9cFixed Codebookxe2x80x9d) segment audio information to re-generate the tones. For more information on this topic, the reader is invited to refer to document xe2x80x9cBaseline text for the IS-127 EVRC TTY/TDD Extension, TR45.5.1.1/99.04.22.16xe2x80x9d The contents of this document is incorporated herein by reference.
One drawbacks of the mode of operation described in xe2x80x9cBaseline text for the IS-127 EVRC TTY/TDD Extension, TR45.5.1.1/99.04.22.16xe2x80x9d is that the data frames containing TTY/TDD messages are sent at a maximum data rate which represents a waste of bandwidth, in particular when the decoder is capable to reconstruct the TTY/TDD characters based only on the information contained in the pitch lag field of the data frames. In addition, since a single Baudot character spans several data frames, a transmission error in a single frame may cause a loss on an entire character. Stated otherwise, to send a single Baudot character, the wireless channel must be error free for all data frames over which the Baudot character is spread.
The background information provided above clearly indicates that there exists a need in the industry to improve the prior art TTY/TDD wireless transmission system such as to provide a more efficient use of bandwidth.
In one broad aspect the invention provides a wireless communication device capable to receive and transmit TTY/TDD messages. The wireless communication device has a receiver that includes an input for receiving an RF signal from a remote site containing a succession of data frames transporting speech sound information or a TTY/TDD message. The receiver is capable to acquire a TTY/TDD active mode when the data frames convey a TTY/TDD message and a TTY/TDD inactive mode when the data frames convey speech sound information. When the receiver operates under the TTY/TDD active mode it is capable to acquire either one of a first operative state in which the receiver processes the data frames at a first data rate and a second operative state in which the receiver processes data frames at a second data rate that is lower than the first data rate.
When the receiver operates in the TTY/TDD active mode the receiver processes the data frames to generate an output signal conveying successive characters of the TTY/TDD message. The receiver, when in the TTY/TDD active mode, is responsive to a TTY/TDD silence message contained in at least one data frame that directs the receiver to blank the output signal from any TTY/TDD character, to acquire the second operative mode and thus process data frames at the second lower data rate. A lower bit rate operation represents a more efficient use of the available bandwidth and this represents an advantage over fixed data rate wireless TTY/TDD systems. In addition, the use of a TTY/TDD silence message as a control message to cause an operative state switch has been found advantageous since it does not create any output and thus goes unnoticed by the user.
In a specific non-limiting example of implementation, the wireless communication device is a cellular telephone that receives data frames from a base station in a cellular network. When operating in the TTY/TDD active mode the receiver is by default in the first operative state, where the data frame processing is done at a full (maximum) data rate. When a data frame is received conveying a TTY/TDD silence message, which is communicated by setting bits in the pitch lag field of the data frame to a specific sequence, the receiver infers that the transmitter at the remote site is capable to interpret the TTY/TDD characters by reading the pitch lag field, and switches to the second operative state where the processing is done at a data rate that is half the full data rate (hereinafter xe2x80x9chalf data ratexe2x80x9d). The cellular telephone will then send to the base station a TTY/TDD silence message that will cause the receiver in the base station to switch to the half data rate also.
If for some reason either one of the receivers stops receiving data frames with the TTY/TTD information embedded in the pitch lag field, they both default to the full data rate processing state.
In a possible variant, the wireless communication device is located at the base station of the digital cellular network.
In another broad aspect the invention provides a method for processing data frames sent over a wireless channel in accordance with the principles presented earlier.
Yet, in another broad aspect the invention provides a wireless communication device capable to receive and transmit TTY/TDD messages. The wireless communication device has a receiver and a transmitter for receiving an RF signal from a remote site containing a succession of data frames, and transmitting to the remote site an RF signal containing a succession of data frames transporting speech sound information or a TTY/TDD message. The receiver can acquire either one of a first operative state in which the receiver is operative to process the data frames at a first data rate and a second operative state in which the receiver is operative to process data frames at a second data rate that is lower than the first data rate. The receiver is responsive to a TTY/TDD silence message contained in at least one data frame directing the receiver to blank its output signal from any TTY/TDD character, to acquire the second operative state and also to cause the transmitter to issue toward the remote site a TTY/TDD silence message.